I was thinking about whether to build an PV AFDD and GFDD detector by myself that is contactless and only operated using a current transformer (this is only about detection, not circuit disconnect after detection, so no AFCI and GFCI devices).
After some research, I found the MNHBB-AFGF sensor from MidNiteSolar which is doing exactly what I want. Unfortunately, it only works with the corresponding MidNiteSolar MPPT chargers. I haven't been able to find a generic AF/GF detector for PV (DC) yet. Here is a picture of this MidNite module.
After further research, I came across the paper "Arc-Faults Detection in PV Systems by Measuring Pink Noise With Magnetic Sensors". The process doesn't look that complicated and I hope is will be able to be implemented with an Arduino. The Power Spectral Density (PSD) of the current is required to detect an ARC. The PSD should be able to create out of the Power Spectrum (via FFT) and some arithmetic to determine the pink noise. The covered DC voltage and current range from 50V to 300V and 6A to 30A is a perfect fit for typical home PV systems. The ARC detection Frequency is in the range of 0Hz to about 800Hz which should be possible with an Arduino - but maybe not with 8 channels in parallel?
The GF detection could be determined relatively easily, as with the MidNite module, using current delta with 2 CTs (one around PV+ and one around PV-). 2 CT's would then be necessary per PV string to cover both AFDD and GFDD. An Arduino could probably control multiple strings (CT's).
Such a device would be very helpful for people like me who own an all in one (AIO) solar inverter/charger devices that do not have integrated AF and GF detection/disconnects. If an AF or GF was detected you could use it to disconnect the PV strings with external mechanisms, e.g. either with remotely controlled contactors or with, for example, Tigo PVRSS remote switches TS4-A-2F.
Instead of CTs, possibly a coreless compact and cheap tunnel magnetoresistance (TMR) which are mentioned in the article and are attached to the PV cables on the insulation, could be used. I think the accuracy of the sensor and also the FFT, PSD calculation resolution needs not to be very exact and fast to just achieve an ARC detection - but I'm not sure.
In addition to the software challenges (FFT, PSD etc.), I see two additional potential problems:
- With non-isolated HF inverters there is an AC-like ripple signal (50/60 Hz depending on country) where the PV voltage "rides" on top of it (floating), which could interfere with the determination of pink noise.
- Typical PVRSS systems using communication over powerline to control the remote switches, which itself is coupled into the PV line using CTs. This signal could overlap with the ARC detection and possibly have a negative impact on the detection.
I have 8 PV strings and it would be great if I could implement AF and GF detection on the PV side in this relatively cheap (simple?) way with 16 CT's or 16 TMR sensors.
DC ARC faults are very dangerous and usually ending up in fires! DC ARC faults at higher voltages are even more problematic.
Has anyone ever heard of a project like this?
Does someone used coreless TMR sensors, attached to the insulation of a wire, instead of CT's to measure current?
Have someone used an FFT library together with PSD calculation?
Thanks in advance for hints.
